Why is NaN shown after fft is keyed in the command window?

I am trying to calculate the Fast Fourier Transform (fft) using matlab.

CSV file was successfully read. However, NaN appeared after I keyed 'p = abs(fft(signaldata,N))/N);' into the command window.

Furthermore, I have issues when plotting the graph.

plot(fv,p)

where fv = 1x513, and p = 1024x2.

The following error was prompted.

"Error using plot

Vectors must be the same lengths."

I understand that the matrix of the two variables are imbalance.

Can anyone advice what should I do to solve the problems above? Thanks in advance!

Answer 1

[dpb]

Probably there's a missing value in the .csv file that was imported as NaN. What does

all(isfinite(signaldata(:)))

return? If it's not '1' (True) then the above supposition is correct and those NaN values are what are propogated thru the the result. Find out how many and where they are...

sum(isnan(signaldata(:)))
[r,c]=find(isnan(signaldata))

Presuming it's only an end effect or somesuch, remove these rows--

signaldata(r,:)=[];

See

doc fft

for an example of how to do the PSD and the frequency vector from the double-sided fft result returned by Matlab to solve your length issues. After the above you may want to pad or fix the input data stream first.